The invention relates to a system and method for controlling ammonia injection upstream of a selective reduction catalyst for use with an internal combustion engine.
In order to meet some emission regulations, selective catalytic reduction systems using externally added reducing agents may be used. In such a system, regulated emissions, such as certain nitrogen oxides, or NOx, can be reduced in a oxygen-rich environment to nitrogen and water over a catalyst when a reducing agent, such as ammonia, is added. In addition to controlling nitrogen oxide emissions, the amount of excess ammonia, or ammonia slip, must be managed. Ammonia slip is experienced when ammonia in excess of that used to reduce the nitrogen oxides passes through the catalyst unaffected and exits the catalyst (as ammonia slip).
One method for regulating ammonia slip is to use an ammonia sensor located downstream of the catalyst. The detected ammonia concentration is compared with a fixed upper threshold value. This comparison generates a correction signal that is used to control the metering of ammonia upstream of the catalyst. Allegedly, by regulating actual ammonia slip to the upper threshold value, a certain nitrogen oxide reduction is obtained. Such a system is disclosed in U.S. Pat. No. 5,369,956.
The inventors herein have recognized a disadvantage with the above system. The above system regulates to a fixed concentration value for the upper threshold ammonia slip. However, this system does not consider NOx conversion efficiency or percentage slip. While NH3 slip expressed as concentration (ppm) and as a percent are related, there is an important distinction in their use for reductant control strategy. In general, as maximum NOx conversion is approached with increasing ammonia addition (i.e., increasing NH3/NOx mole ratio), ammonia starts to slip. After maximum NOx conversion is attained, ammonia slip increases more rapidly with increasing NH3/NOx. For example, if ammonia slip is regulated to a constant concentration value, an ammonia setting high enough for sufficient NOx conversion at high NOx feed gas levels is likely excessive for low NOx feed gas levels, thereby wasting ammonia. Conversely, a setting at minimum detectable ammonia concentration is likely insufficient to provide high NOx conversion at high NOx feed gas levels. Further, intermediate settings may still be insufficient to provide high enough NOx conversion at high NOx feed gas levels. Thus, prior approaches can not achieve high NOx conversion with minimal ammonia slip, particularly for vehicle engines where NOx concentration levels varies widely and quickly.
In other words, because a catalyst experiences widely varying levels of engine NOx, controlling to an ammonia slip concentration results in widely varying, and less than optimum, NOx conversion efficiency.
An object of the invention claimed herein is to provide a system and method for controlling ammonia injection upstream of a selective reduction catalyst using an ammonia sensor located downstream of the catalyst to keep ammonia slip low while achieving a high level of NOx conversion.
The above object is achieved and disadvantages of prior approaches overcome by a method for controlling a reductant injection into a catalyst coupled to an internal combustion engine, the method comprising the steps of: determining a temperature region in which the catalyst is operating; generating a reductant injection quantity based on engine operating conditions; generating a desired reductant slip based on a catalyst temperature and said reductant injection quantity; and adjusting said reductant injection quantity so that an actual reductant slip approaches said desired reductant slip.
By regulating reductant slip to a desired value that is a fraction of injected reductant, NOx conversion efficiency is kept high and more consistent throughout widely varying NOx concentration levels typical for diesel vehicles. Further, since the desired ammonia slip value is also based on temperature, this additionally improves NOx conversion.
It is therefore possible to control ammonia slip with improved NOx reduction, particularly for vehicle engines where NOx concentration levels varies widely and quickly. In other words, when ammonia slip is regulated to a fraction of injected reductant, or ammonia, high NOx conversion is provided without excessive slip throughout the widely varying NOx feed gas concentrations.
An advantage of the present invention is improved NOx conversion while keeping ammonia slip low.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.